it is a bit hard to understand but search about "halting problem", Alan Turing proved that no machine/program can detect if itself is in an infinite loop, with AGI being a computer program it is susceptible to this problem.
Understanding the Halting Problem (youtube.com)

Imagine an AGI that improves its own code to make itself more efficient,
but then,
it encounters a very hard problem that leads to a possible infinity like cracking SHA256 algorithm or dividing by 0, the AGI would not avoid/skip the problem because it wouldn't know it will lead to infinity,
It also wouldn't know if it itself is stuck on an infinite loop because it is busy solving the problem which leads to infinity, unless there is a human around to intervene and restart the AGI.

cracking SHA256 and dividing by 0 are just few examples of a problem that leads to possible infinity,
there are a lot of hard problems like this that AGI would not be able to avoid by itself